High levels of absorption in orientation-unbiased, radio-selected 3CR Active Galaxies

Belinda J. Wilkes, Martin Haas, Peter Barthel, Christian Leipski, Joanna Kuraszkiewicz, Diana Worrall, Mark Birkinshaw, Steven P. Willner



A critical problem in understanding active galaxies (AGN) is the separation of intrinsic physical differences from observed differences that are due to orientation. Obscuration of the active nucleus is anisotropic and strongly frequency dependent leading to complex selection effects for observations in most wavebands. These can only be quantified using a sample that is sufficiently unbiased to test orientation effects. Low-frequency radio emission is one way to select a close-to orientation-unbiased sample, albeit limited to the minority of AGN with strong radio emission.Recent Chandra, Spitzer and Herschel observations combined with multi-wavelength data for a complete sample of high-redshift (1 24.2) = 2.5:1.4:1 in these high-luminosity (log L(0.3-8keV) ~ 44-46) sources. These ratios are consistent with current expectations based on modelingthe Cosmic X-ray Background. A strong correlation with radio orientation constrains the geometry of the obscuring disk/torus to have a ~60 degree opening angle and ~12 degree Compton-thick cross-section. The deduced ~50% obscured fraction of the population contrasts with typical estimates of ~20% obscured in optically- and X-ray-selected high-luminosity samples. Once the primary nuclear emission is obscured, AGN X-ray spectra are frequently dominated by unobscured non-nuclear or scattered nuclear emission which cannot be distinguished from direct nuclear emission with a lower obscuration level unless high quality data is available. As a result, both the level of obscuration and the estimated instrinsic luminosities of highly-obscured AGN are likely to be significantly (*10-1000) underestimated for 25-50% of the population. This may explain the lower obscured fractions reported for optical and X-ray samples which have no independent measure of the AGN luminosity. Correcting AGN samples for these underestimated luminosities would result in flatter derived luminosity functions and potentially change their evolution.
Originele taal-2English
StatusPublished - aug.-2014
EvenementAmerican Astronomical Society, HEAD meeting #14, #400.03 - Chicago, United States
Duur: 17-aug.-201421-aug.-2014


ConferenceAmerican Astronomical Society, HEAD meeting #14, #400.03
Land/RegioUnited States


Duik in de onderzoeksthema's van 'High levels of absorption in orientation-unbiased, radio-selected 3CR Active Galaxies'. Samen vormen ze een unieke vingerafdruk.

Citeer dit